In motor vehicles, there has been increasing use of automatic stepped transmissions having at least one automated friction clutch as a startup and shifting clutch, in which the gear selection, the triggering of shift operations, the engaging and disengaging of gear steps, and the engaging and disengaging of the friction clutch are automated; that is, these actions occur by evaluating operating parameters in a transmission control device and the drive assigned to the control.
Particularly in the case of commercial vehicles, the drive engines are usually designed as diesel engines that can be boosted by a turbo-charger and that have a specific load build-up characteristic. As described in more detail in the document DE 10 2008 054 802.2, which was previously unpublished and which discloses a method for controlling an automatic stepped transmission depending on the dynamic operating characteristics of a turbo-charged internal combustion engine, a turbo-charged internal combustion engine can spontaneously, that is with high torque gradients, only reach an intake torque lying below the full load torque. A further increase of the engine torque is briefly possible, although with low torque gradients, only above a boost threshold speed, above which the turbo-charger creates a significant increase of the charge pressure and thus the engine torque. Thus aside from the idle speed, cut-off speed and the full load torque characteristic curve, the dynamic behavior of a turbo-charged internal combustion engine is also determined by the boost threshold speed and the intake torque characteristic curve as well as by the torque gradients present in certain ranges. Due to the limitation of the spontaneously achievable engine torque to the intake torque, with turbo-charged internal combustion engines, significant torque deficiency, generally referred to as turbo lag, is observed below the boost threshold speed when the power that is requested by the driver by deflecting the gas pedal requires engine torque that is greater than the intake torque.
To avoid or at least mitigate the undesired turbo lag, multiple technical solutions were disclosed such as an adjustable turbine geometry for improving the response behavior of the exhaust gas turbo-charger, or auxiliary devices for increasing the charge pressure at low engine speed, for instance a mechanically drivable compressor, an electrically drivable supplemental compressor, or a mechanical or electrical drive of the drive shaft of the exhaust gas turbo-charger. However, such devices are relatively complex and expensive, increase the construction space requirements and represent increased failure potential for the operation of the internal combustion engine, so they are frequently omitted.
Particularly in the case of a loaded commercial vehicle traveling uphill, which is typically performed at high engine load, that is, at an engine torque lying above the intake torque, a travel situation can occur with torque deficiency of the drive engine, for example when the driver briefly releases the gas pedal or significantly reduces the gas pedal setting to avoid a collision with a slower vehicle traveling in front for example. If the gas pedal setting and thus the power demanded by the driver is then significantly increased again, for example because slower vehicle traveling in front has turned off or can be overtaken, the charge pressure and the engine speed of the drive engine can have been reduced so far that the drive engine can no longer spontaneously attain the previously set high engine torque, however the briefly attainable maximum intake torque is not sufficient for overcoming the drive resistance.
The torque deficiency of the drive engine in this travel situation can be remedied either by shifting into a lower gear, or by startup from a standstill or from a slow rolling speed. For downshifting, a lower gear must be available however, which particularly in the case of a lighter weight commercial vehicle is frequently not the case due to a low number of gears of the respective stepped transmission. In addition the drive resistance (rolling resistance+incline resistance) must not be too high, because otherwise the motor vehicle is decelerated too greatly during the shift-dependent interruption of tractive force, and then startup from standstill is necessary in any case. However, startup from standstill is associated with a loss of comfort and with high thermal and mechanical loading of the friction clutch, and under certain circumstances, such as travel on difficult terrain, may no longer be possible.
A further driving situation with torque deficiency of the drive engine can arise during travel with low engine load and low engine speed, when the driver wishes to accelerate but the spontaneously attainable intake torque of the drive engine is not sufficient for this purpose, that is, no acceleration is possible (intake torque=drive resistance torque) or only very small acceleration (very low excess torque compared to the intake torque available for acceleration) is possible. The torque deficiency of the drive engine can be remedied in this driving situation too by downshifting, but at the cost of the aforementioned risks and disadvantages.
Many suitable methods have already been proposed for overcoming torque deficiency of a turbo-charged internal combustion engine occurring in other operating situations of a motor vehicle. Thus for example, the document DE 102 34 428 A1 discloses an appropriate method for the startup control of a motor vehicle, the drive train of which comprises a drive engine built as a turbo-charged internal combustion engine, a startup element built as a hydrodynamic torque converter and a transmission built as a planetary automatic transmission. This known method provides that during a startup procedure a load carrying friction shift element (clutch or brake) of the automatic transmission is operated with slip for a brief period of time such that the internal combustion engine can build-up increased startup torque using an increased engine speed.
The document U.S. Pat. No. 6,692,406 B2 describes a corresponding method for the gearshift control of a motor vehicle, the drive train of which comprises a drive engine designed as a turbo-charged internal combustion engine, a startup and shifting clutch designed as an automated friction clutch, and a transmission designed as an automatic stepped transmission. This known method provides that with an upshift at full load, the internal combustion engine is controlled such that the charge pressure is maintained during the shifting procedure either by increasing the exhaust energy or by maintaining the rotational speed of the exhaust gas turbo-charger, and thus sufficiently high engine torque can be built up at the end of the shifting procedure.
Due to different operating situations and other technical conditions, the two named methods cannot however be readily applied to the present stated problem.